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Subject: Re: [PATCH] serial: sh-sci: Use spin_{try}lock_irqsave instead of
 open coding version
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-rt-users@vger.kernel.org,
        "open list:SERIAL DRIVERS" <linux-serial@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linux-sh list <linux-sh@vger.kernel.org>,
        Daniel Wagner <daniel.wagner@siemens.com>,
        Shinya Kuribayashi <shinya.kuribayashi.px@renesas.com>
References: <20180430080918.16763-1-wagi@monom.org>
 <CAMuHMdUjsy95zNtoZnp17dmgRcrkzfSCxTe5G=ho5nphqX0HEg@mail.gmail.com>
 <20180507124704.s4qlrcc3leoky4r7@linutronix.de>
From:   Daniel Wagner <wagi@monom.org>
Message-ID: <66cd1363-9f62-e2f5-2271-1fa78509f2cd@monom.org>
Date:   Tue, 8 May 2018 09:23:57 +0200
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On 05/07/2018 02:47 PM, Sebastian Andrzej Siewior wrote:
> On 2018-05-03 09:43:33 [+0200], Geert Uytterhoeven wrote:
>>> --- a/drivers/tty/serial/sh-sci.c
>>> +++ b/drivers/tty/serial/sh-sci.c
>>> @@ -2516,13 +2516,12 @@ static void serial_console_write(struct console *co, const char *s,
>>>          unsigned long flags;
>>>          int locked = 1;
>>>
>>> -       local_irq_save(flags);
>>
>> Hence the below now runs with local interrupts enabled.
>>
>> For checking port->sysrq or oops_in_progress that probably isn't an issue.
>> If oops_in_progress is set, you have other problems, and the race condition
>> between checking the flag and calling spin_lock{,_irqsave}() existed before,
>> and is hard to avoid.
> 
> while oops_in_progress is an issue of its own, the port->sysrq isn't
> avoided by by local_irq_save(). On SMP systems you can still receive a
> `break' signal on the UART and have a `printk()' issued on another CPU.
> 
>> For actual console printing, I think you want to keep interrupts disabled.
> 
> why? They should be disabled as part of getting the lock and not for any
> other reason.
> 
>>>          if (port->sysrq)
>>>                  locked = 0;
>>>          else if (oops_in_progress)
>>> -               locked = spin_trylock(&port->lock);
>>> +               locked = spin_trylock_irqsave(&port->lock, flags);
>>>          else
>>> -               spin_lock(&port->lock);
>>> +               spin_lock_irqsave(&port->lock, flags);
>>
>> Add
>>
>>          if (!locked
>>                  local_irq_save(flags)
>>
>> here?
> 
> So for oops_in_progress you get here with interrupts disabled. And if
> not, I don't see the point in disabling the interrupts without any kind
> of locking.

So I understand, the initial version of this patch was correct.

@Geert if you don't object I'll send a v3 (v1 ported to mainline).

Thanks,
Daniel